Anal. Chem. 1988, 60, 1684-1694
1684
The Photodetachment-Modulated Pulsed Electron Capture Detector. Iodide- and Bromide-Specific Detection for the Trace Analysis of Halocarbon Mixtures. Photodetachment Cross Sections of the Halides R. S. Mock and E. P. Grimsrud* Department of Chemistry, Montana State University, Bozeman, Montana 5971 7
The use of photodetachment (PD) of electrons from negative Ions In a pulsed electron capture detector (ECD) Is described. Sensltlve responses to halogenated hydrocarbons that produce either I-, Br-, or CI- upon electron capture can be created by passing a chopped llght beam through the ECD and ampllfylng the modulated component of the ECD signal. By use of a relatively simple llght source system, Including an arc lamp and a hlgh-throughput monochromator, the photodetachment-modulated (PDM) pulsed ECD can be made to respond selectlvely and sensitlvely to lodlne-contalnlng hydrocarbons alone or to iodine- and bromine-contalnlng hydrocarbons in the presence of chlorinated hydrocarbons. Thls capablllty Is shown to be useful In the trace analysls of a complex mlxture of halogenated hydrocarbons by gas chromatograpy. Slmultaneous wlth this PDM response, a normal ECD response Is also obtained. The ratio of the modulated and normal responses thereby obtained during a single chromatogram provldes an indlcatlon, along wlth the chromatographic retentlon t h e , of the Identity of the analyte. The detection llmlt of the lodlde/bromlde-speclflc mode of the PDM-ECD to CH,I Is shown to be competitlve with that of the normal mode of the pulsed ECD. I t Is shown that events occurrlng wlthln thls detector can be modeled with sufflclent accuracy as to allow reallstlc predlctlons of the PDM-ECD responses to be made, If the PD cross sectlons of the negative ions produced by electron capture are known. Conversely, It Is shown that the PDM-ECD provldes a means by which relatlve and even absolute PD cross sections can be measured. The photodetachment spectra of CI-, Br-, and Iand the absolute PD cross sectlon for I- at 365 nm are reported here and are shown to be In excellent agreement with prevlous measurements by other methods.
The electron capture detector (ECD) is undoubtedly the most commonly used detector for the trace analysis of environmental pollutants by gas chromatography. This popularity results primarily from the fact that the gas-phase electron attachment reactions of numerous molecules of environmental interest are extremely fast, while the attachment rates of many hydrocarbons that commonly constitute an uninteresting and large fraction of an environmental sample are slow. In favorable cases, therefore, it is possible to analyze real environmental samples with a minimum of preliminary sample cleanup. For example, the analysis of halocarbons in relatively clean air is easily performed by GC-ECD by injection of the whole air sample into the GC with absolutely no need for prior removal of coeluting hydrocarbons that are present in huge excess (1-3). If the sample is more complex, however, and contains tens or hundreds of organic molecules that have significant electron capture coefficients, the level of specificity
inherent in the ECD may be insufficient to provide a reliable determination of the sought-after substances. In these cases, extensive sample cleanup and/or use of an even more selective (and more expensive) detector, such as a mass spectrometer, may be required. In recent years the versatility of the ECD for broad-range analysis has been increased by a series of discoveries that might be collectively termed the chemically sensitized (CS) ECD. By the intentional addition of oxygen (4,5)or nitrous oxide (6) to the carrier gas, it has been shown that an ECD can then be made to respond to various classes of molecules that do not attach electrons rapidly under normal conditions. Also, by the intentional addition of ethyl chloride to the carrier gas, it has been shown that the ECD's response to low-electron-affinity compounds, such as anthracene, is greatly improved (7). While the CS-ECD will be very useful in expanding the general utility of the ECD, it is not clear whether it will be of assistance with the problems associated with samples that are too complex for analysis by GC-ECD. What is needed in order to improve the ECD's applicability to very complex samples is an induced perturbation of the ECD response which, when detected and amplified, provides an additional element of response specificity toward those components of the sample that are of interest. In this paper we will explore the use of light-induced photodetachment (PD), reaction 1, for this purpose. M- hv M e(1) Ideally, this electron capture/photcdetachment detector would respond with the same high level of sensitivity normally achieved by the ECD, but only to molecules that (1)attach thermalized electrons and (2) lead to negative ions which readily undergo photodetachment by a selected beam of light. Photodetachment spectra of numerous atomic (8-13) and polyatomic (14-21) negative ions have been previously reported. Several review articles on this topic have also been written (22-26). The PD process is a transition from a bound electron in the anion to a free continuum electron plus the neutral. The minimum energy required of the photon in order to induce PD is equal to the electron affinity (EA) of the neutral. Since electron affinities seldom exceed 4 eV, visible and near-UV light sources can be used to induce P D of negative ions in common matrix gases. Photodetachment spectra are often broad and relatively featureless. For atomic negative ions, such as the halides shown in Figure 1, the increase in cross section, u, with photon energy is very abrupt in the region of the EA-determined threshold. The most significant difference between the PD spectra of I-, Br-, and C1- shown in Figure 1 is the differing onsets of P D corresponding to the EA of each halide atom. The PD spectra of polyatomic anions tend to exhibit a much more gradual increase in c with increase in photon energy in the threshold region. This result is expected due to the added complexities associated with the numerous vibronic and rotational states that are accessible
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0003-2700/88/0360-1684$01.50/00 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60,NO. 17,SEPTEMBER 1, 1988
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10.
51
8.
6-
4-
0
200
400
k,
400
360
320
280
240
wavelength ( n m ) Figure 1. Photodetachment spectra and cross sections of iodide, bromide, and chloride negative ions (reproduced from ref 8 and 9). Also shown are the emission spectra of a Xe (dashed line) and a Hg-Xe (dotted line) arc lamp.
600
8 00
(sec-')
Flgure 2. Theoretically predicted effects of photodetachment and its associated rate coefficient, khu,on the 61N, 6IL, 6JL,c, and 61Mf responses of the pulsed ECD. Calculations were made from a model that includes reactions 2-6, for which the following rate coefficients have been used: 0 = 1.87 X 10" ion pairs s-'; Ran+ = 300 s-'; R,-n, = 100 s-'. While kenw = 100 s-' and a pulse frequency of 2 kHz are used here, predictions of relative responses are independent of the magnitudes chosen for these parameters.
to the polyatomic negative ion and the neutral product. Also,
differences in preferred geometries of the negative ion and the product neutral (i.e. poor Franck-Condon overlap) can greatly complicate the P D spectra of polyatomic anions even to the point where, as in the case of SF, (14),no P D may be observed with use of light several times more energetic than the EA. In the above studies of PD, measurements were generally made by one of two general approaches. In one of these, the absorption of light passed through a shock tube containing an extremely high density of ions is measured. The other general approach involves measurement of the diminution by light of mass spectrometrically generated ion beams. The measurement of a PD spectrum by an ECD has been described on one previous occasion. In that report by Dovichi and Keller (27), a line-tunable argon-ion laser producing up to 3 W of continuous luminous power in the selected wavelengths was used in conjunction with a direct current (DC) ECD. with this instrument, PD-modulated (PDM) ECD responses to NO2 were observed. Although the PDM signals were weak, they were sufficiently strong as to provide a PD spectrum of NO, that was consistent with the known P D spectra of that negative ion. While the report of Dovichi and Keller demonstrated that P D spectra can be obtained by use of an ECD, the level of sensitivity demonstrated in that study was not encouraging with respect to the potential use of PD in an ECD for trace organic analysis. In their study, low PDM-ECD sensitivity can be partially attributed to the small cross section for P D of NOz- a t the wavelengths used; a t 488 nm, u = 4 X cm2 (21). Also, low sensitivity may have resulted from their use of a DC-ECD. The basic operational principles of a DC-ECD are not well understood (28). In particular, the location and lifetimes of negative ions within a DC-ECD are unknown. In this paper we wish to report the use of P D in an ECD that is of the pulsed design. With a pulsed ECD it is possible to more clearly describe the dynamics occurring within its ionization volume. Several of the details of this description point to potential advantages of the pulsed ECD over the DC-ECD for its application with PD. For example, it is known that the negative ions within a pulsed ECD will be relatively long-lived and will be concentrated by a positive ion space-
0
P 400
0
800
800
200
Flgure 3. Predicted relationship between the ratio of the 61Mf and 61, responses and khu. Rate coefficients used are those listed in Figure 2, except the values chosen for the negative ion recombination coefficient, R,-n+, are (A) 100,(B) 50, and (C) 150 s-I. recorder /
:'"
I
I \
chromatograph
monochromator
Figure 4. Apparatus used for photodetachment-pulsed ECD measurements.
charge field within specific regions of the ionization volume (29). The location of this region is predictable and can be tailored to suit preferences by appropriate design of the cell (30). In the study to be reported here, a cylindrical cell will
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
B
A
I
0.3
I
I
0.3.
i H
0.2.
0.1. h
0
-
02
C
0,
0.4.
98
IC
(relative)
Figure 6. Ratio of 6IM/6IL,c responses to (A) CH,I, (B) CF,Br,, and (C) CHCl3 observed with use of various relative intensities of light emitted by the Xe arc lamp, without use of a monochromator or filters. Solid lines are predictions of relative response ratios based on the
0.3-
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04
H
overlap integrals of the PD cross sections and the Xe lamp emission spectrum shown in Figure 1.
0.2.
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I
Oo3i
CHC13 0
4
C
5
time ( m i n ) Figure 5. Chromatograms obtained from analysis using the four different response functions possible with the use of light in a pulsed ECD. Chromatograms in column A were obtained with use of the full emission of a Xe arc lamp (40% of maximum power); those in column B, with the full emission of a Hg-Xe arc lamp. The first three response functions shown were obtained by the usual pulsed ECD mode: (1) with the lamp turned off, (2) with light continuously passing through the ECD, and (3)with a chopped light beam of 43 Hz passing through the ECD. Bottom chromatograms where obtained simultaneously with the third chromatograms by detection and amplification of the 43-H~ PM-modulated component of the pulsed ECD signal. be used, in which the negative ions formed by EC will be contained along its central axis (30). Since a light beam will also be passed through this region of space, the probability of photon-negative ion encounters will be maximized. Since most of the relevant processes that affect the current measured with a pulsed ECD can be modeled with a useful level of accuracy (29, 31,32),a relatively detailed understanding of the quantitative response of the PDM-pulsed ECD should be possible. This would be useful, of course, in its applications for chemical analysis. Furthermore, through this understanding, it may be possible to determine the absolute, as well as relative, PD cross sections of negative ions from PD-pulsed ECD measurements.
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, 33
, 43
, 53
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.
,
,
,
73
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,
chopper frequency ( Hz) Figure 7. Observed 61,161, response ratios and phase angle offsets in the chromatographic analysis of CHJ, CHCI,, and CCI, as a function
of chopper frequency.
The compounds chosen for study here are various halogenated hydrocarbons that are known to produce C1-, Br-, or I- upon electron capture. These anions are commonly produced in analysis by the ECD and, therefore, the results
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17;SEPTEMBER
/
i
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400
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320
340
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wavelength ( n m ) Flgure 10. Measured PD spectra of (A) iodide, (B) bromide, and (C) chloride. 6 1 , and 6I,,, response ratios observed for CH31, CF,Br,, and CHCI, and measured relative l i i t flux, a, are plotted in a functional form that is expected to be proportlonal to the PD cross sections at each Wavelength.
C
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CClzFCClF2 CHC13 8 C2Hg
a
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0
5
0
5
0
5
t i m e (min.) Figure 9, 6I,, and 6IM responses simultaneously observed In the repeated analysis of a sample containing CF,Br,, CH,I, and CHCI,. Along with the Xe arc lamp, a monochromator has been used and set to (A) 330 nm, (B) 365 nm, (C) 395 nm, and (D) 425 nm.
reported here will be applicable to the analysis by the ECD of hundreds of other compounds. The photodetachment spectra of the halides have been previously measured by other methods (8-10,12). The most recent measurements, by Mandl (8,9),shown in Figure 1,indicate that the PD cross sections of the halide anions are relatively large, on the order of (1-3) X em2,over the wavelength range shown. Therefore, the group of compounds chosen for study here will allow testing of the PDM-pulsed ECD on a favorable, as well as analytically important, chemical system. Since the cross sections of the halides are known, the experimental results reported here can be compared against theoretically predicted responses.
THEORY With use of a pulsed ECD in conjunction with PD, it is possible to construct a model of the detector that will be of assistance in predicting and understanding the magnitude of its qualitative responses to various molecules. The model to be presented here involves the addition of one reaction, that due to photodetachment, to models that have been previously developed for the pulsed ECD (29,31). In a clean detector
0 5 10 min. Figure 11. Chromatographic analysis of a mixture of 11 halogenated hydrocarbons with simultaneous detection by the 6 I and 6 I modes of the PDM-pulsed ECD. The top chromatogram provides responses to ail compounds present, while the lower chromatogram provides responses only to the bromides and iodides. The light used is of wavelength 365 nm and was obtained from the Hg-Xe arc lamp. The concentrations of the 11 halocarbons, in the order shown in the upper chromatogram, are as follows: 1.3, 2.0, 0.6, 25, 8.5, 10, 19, 2.4, 900, 3.3, and 15 ppb in nitrogen.
,,,
that contains nitrogen or argon-methane carrier gas, the analyte (MX), and a beam of light, the processes that should adequately explain PD perturbations of an ECD response are shown as reactions 2-6. the processes shown are expected to determine the magnitude of the electron population achieved during the period of time between the application of electron-removing pulses to the ECD anode.
N2
B
P+ + e-
(2)
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
e-
+ P+
Re
- + -+ ke
+ MX Rx X- + P+
e--
X-
+ hv
neutrals
khu
X-
M
(3)
(4)
neutrals
X
e-
In reaction 2 the carrier gas is ionized by beta radiation to form a set of positive ions P+ and an equal number of electrons. The rate coefficient p for this process is found by measurement of the maximum standing current obtained in the absence of sample and with use of a very rapid pulse frequency (33). For our cell, = 1.87 X 1O'O ion pairs s-l. In reaction 3 the recombination of electrons with positive ions is shown. For a given detector the product of the rate coefficient, Re, and the positive ion density, n+, can be determined from measurements of standing current as a function of pulse frequency (33). For our detector at 100 "C, Ren+is found to be 300 s-l. Reaction 4 shows dissociative electron capture by the analyte to form the negative ion, X-. The organic halides to be studied here are known to capture electrons by this process to form either C1-, Br-, or I- (31). The first-order rate of electron loss by this process will be given by the product, kenMX* The recombination of negative ions with positive ions is shown in reaction 5. The psuedo-first-order rate coefficient for this process, Rx-n+,will be of central importance in predicting the magnitude of PD perturbations of the ECD response because reaction 5 and reaction 6 are thought to directly compete for the available negative ions, X-. If destruction of X- occurs by reaction 5 , only neutrals are formed, and the loss of an electron from the system caused by reaction 4 will be recorded by the electrometer. If, however, X- is destroyed by reaction 6, the electron is regenerated and the electron capture event is then not recorded by the electrometer. In the following two paragraphs, individual consideration of the probable magnitudes of the rate coefficients for reactions 5 and 6 is given. The rate coefficient Rx-n+for reaction 5 cannot be measured directly by an ECD. However, an estimate of its magnitude is possible, as will now be described, from the measurement of Ren+ discussed above for reaction 3. In prior studies of atmospheric pressure ionization in a "Ni ion source, an increase in total positive ion density has been consistently observed (5, 34) whenever the source is altered from an electron-dominated to a negative-ion-dominated system by the introduction of a high concentration of an electron-capturing substance. In our laboratory, the magnitude of this increase in total positive ion signal has been consistently observed to be between 50 and 100%. This effect appears to be independent of the choice of electron-capturing substance used. Since the density of positive ions within a field-free, atmospheric pressure ion source is thought to be determined by recombination reactions, the above observations are thought to reflect the relative magnitudes of Re and Rx-. More precisely, the total positive ion denisities would be expected to be inversely proportional to the square root of the recombination coefficients involved (29,34). The above observations then lead to the following estimate: Rx- is expected to be about one-half to one-fourth as great as Re. This assessment of the relative magnitudes of Rx-and Re is consistent with other data concerning this subject. At atmospheric pressure, ion-ion recombination coefficients tend to a constant value of about 1 x lo4 cm3 s-l (35). Positive ion-electron recombination coefficients, on the other hand, can exceed this value if the positive ions are cluster ions. For example, the electron
recombination coefficients of the cluster ions, H+(H20),and H+(H20)3,are 2 X lo4 and 4 X lo4 cm3 s-l, respectively (36, 37). Cluster ions of the type H+(H20),are commonly observed in atmospheric pressure ion sources (38),and the two indicated above are the ones expected in relatively dry carrier gas. In view of these literature values and our observations summarized above, the following assessment seems reasonable: when a small amount of MX enters the ionization cell, the small population of X- that is created (the predominant negative charge carriers are electrons as long as the sample size is kept small) will recombine with positive ions at a rate described by Rx-n+ = 'I3Ren,. Since Ren+ in our cell is found by measurement to be 300 s-l, a reasonable approximation of Rx-n, is thought to be 100 A 50 s-l. The rate coefficient for reaction 6 is given by khv = u@, where u is the cross section for PD and @ is the intensity of the light flux. For example, if a negative ion, such as I- in Figure 1,has u = lo-'' cm2 and is irradiated with 1.0 W cm-2 of 380-nm light (@= 2.0 X lo4 photons cm-2 s-l), then khv= 40 sd. By comparison of this khuvalue with Rx-n+ = 100 s-l, it is seen that with this light intensity, reaction 6 will compete significantly with reaction 5 and will convert almost one-third of the negative ions, X-, to free electrons. This magnitude of PD perturbation on an ECD response should be very easily measured. In a flux of 5 W cm-2 could be passed through an ECD, in the above example, the resulting khv = 200 would cause the majority of the negative ions to be photodetached. This would be expected to cause a huge perturbation of the ECD response. The ECD response perturbations expected to be caused by PD in our detector are shown in Figure 2 as a function of the magnitude of khv = u@. The magnitudes of kh,,values shown in this figure are relatively large, but as described in the previous paragraph, the lower portion, a t last, of the range shown should be achievable for favorable systems. Four different types of ECD responses are represented in Figure 2, each of which is obtainable with the instrumentation described in the Experimental Section. SI, is the normal ECD response expected in the absence of light. 61, is the ECD response expected when light is continuously passed through the cell. SIM' is the PD-modulated component of the ECD response expected to be cause by a chopped light beam. In the calculation of S I M ' , it is assumed that the dynamic processes within the ECD are very fast on the time scale established by the beam chopper. Because of this assumption, the real measurement, S I M , will not necessarily be equal to SIM'. The expected relation between SI, and SI,' will be considered later. It is seen in Figure 2 that 61,' = S I N - SI,. SILjc is the normal ECD response expected when a chopped light beam is passed through the cell. It is seen that 61Llc is equal to the average of S I N and SI, and closely resembles S I N for small values of khu. All of these predictions of responses have been calculated with a personal computer by numeric integration of the processes described in reactions 2-6 using the rate coefficientsdiscussed above and those indicated in the caption of Figure 2. Boundary conditions used in these calculations are that the population of electrons grows during each period between electron-removing pulses from zero to some positive value and that the population of negative ions is the same at the beginning and the end of each period between pulses. In these calculations, the choice of sample size has no effect on the relative responses indicted in Figure 2 as long as sample size is kept small so that no SI responses are greater than 10% of the total standing current. Within this low-sample range all absolute responses are predicted to be proportional to the sample concentration. The predictions of Figure 2 are that over most of the range of khuvalues considered, PD should induce very significant
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
and easily measured perturbations of ECD responses. In particular, it is expected that the magnitude of the modulated response, 6ZM’, can easily be made as great as 10% of the normal ECD response, SIN. At the higher khu values shown, the PD effect is expected to approach a saturation condition in which further increases in light intensity would be expected to cause smaller additional P D effects. In curve A of Figure 3 the ratio of expected SIM’ to 6ILlc responses is plotted as a function of khu. The ratio of these two responses will be of particular interest because they can be simultaneously and, therefore, conveniently obtained in a chopped-beam experiment. In addition, we expect that a comparison of these two signals will provide more accurate P D information than a comparison of normal ECD signals with light continuously on and light continuously off, because any effects of an intense light beam on source gas temperature (27) will be minimized in a chopped-beam experiment. Figure 3 indicates that the ratio of these responses is expected to be a sensitive function of the rate coefficient khvfor reaction 6 and may provide a means of determining khv and also u if can be independently measured. The accuracy of this method for determining khv or u will depend on the accuracy of our estimate for Rx-n,, which as discussed above is thought to be given by Rx-n, = 100 f 50 s-l. Therefore, in Figure 3 curves B and C are also shown, for which values of 50 and 150 s-l were used for Rx-n,.
EXPERIMENTAL SECTION The experimental setup is shown in Figure 4. The ECD used here was home-built from stainless steel. Its ionization volume is cylindrical, with a length of 2.0 cm and diameter of 1.3 cm. A “Ni-on-Pt foil (New England Nuclear) of 9-mCi activity forms in. diameter pin the cylindrical walls of the cell. A stainless serves as the anode of the cell. It enters the cell from one side, as shown, through a Teflon plug and protrudes only l f g in. into the ionization volume. Both ends of the cell are formed by fused silica windows, which are mechanically held and sealed by use of Teflon washers. The cell is heated by two cartridge heaters with thermocouple-feedback control to temperatures up to 200 “C. Samples are introduced to the ECD by a gas chromatograph (Varian Model 3700) incorporating a 10-ft X ‘f8-in. stainless column packed with 10% SF-96 on Chromosorb W. Nitrogen carrier gas was first passed through water- and oxygen-removing traps with a flow rate of 40 mL min-’. Injection of gaseous sample mixtures was performed by use of a 5.0-cm3gas sampling loop (Carle Model 8030). All optical components were attached to an optical rail. These consisted of a 1OOO-W arc lamp (Photon Technology International (PTI), Model A 5000),an IR water filter (PTI), a mechanical light beam chooper (PTI, Model 4000), and a single-grating monochromator (PTI; blaze angle, 400 nm), the focal length of which is matched to that of the arc lamp. The arc lamp has an elliptical reflector of F/4, which focuses most of its emitted light to a circular image about 1 cm in diameter and about 17 in. from the lamp face. The lamp can produce up to 105 W of total luminous power. With a Xe lamp the emission spectrum is relatively evenly spread over the visible and near-UV wavelengths. When a Hg-Xe lamp is used greater concentrations of light within specific regions of this spectral range can be obtained. The emission characteristics of these two lamps over the wavelengths of interest in this study are shown in Figure 1. The monochromator was used with its entrance and exit slits wide open (5 mm) for maximum light throughput. Under this condition the bandwidth of the monochromator is 20 nm. The five-sector wheel supplied by the manufacturer of our light beam chopper was replaced by a home-built two-sector wheel. With this wheel the width of each sector exceeded the size of the useful light image defied by either the entrance window of the ECD or entrance slit of the monochromator by a factor of about 5. Therefore, the on-off nature of the light beam defined by the chopper closely resembles a square wave with a 50150 duty cycle. The signal processing components in Figure 4 include the ECD pulser and electrometer (home-built), a lock-in amplifier
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(Princeton Applied Research, Model 5207), a two-pen recorder, and the light beam chopper previously described. The basic design of the pulser and electrometer has been previously reported (39). The pulser is of the fixed-frequency type and was set to 2 kHz. The resulting current signal undergoes standard processing (39) by the electrometer circuit and is sent to pen 1 of the recorder and also to the lock-in amplifier as shown. In the lock-in amplifier, the electrometer signal is mixed with the square waveform coming from the chopper, and after the proper phase angle offset of these two waveforms is established by the lock-in amplifier, the resulting product is sent to pen 2 of the recorder. Thus,in a chopped-beam experiment, pen 1provides a measure of the signal, bILlC,which, as described in Theory, is closely related to a normal ECD response. Simultaneously, pen 2 provides a response, SI,, which is the actual PD-modulated ECD response and is closely related to the SI,’ response described in Theory. The relation between SI, and SI,’ is determined by ECD dynamics and the phase angle offset of the lock-in amplifier. This important point will be discussed in detail in Results and Discussion. The other two responses discussed in Theory, SIN and bIL,can each be provided by pen 1 in experiments in which the chopper is not used and the light source is either continuously off or on, respectively. The halomethanes used in this study were obtained in pure form from commercial suppliers. Mixtures of these in nitrogen gas were prepared by successive dilution into gastight glass vessels with final storage in a 4.5-L glass carboy. This carboy was pressurized slightly with nitrogen gas, allowing numerous aliquots to be transferred by 50-mL syringe to the 5-mL sampling loop of the gas chromatograph. The reproducibility of sample delivery by this method is very high (
